What is the Main Feed? What's Latest Activity?

The first one that you see when you click on the home page is your Main Feed which consists of the activity of anyone you're friends with or follow.

The second one is the Latest Activity feed, the activity you are doing yourself (commenting on posts and pictures, uploading writing, pics, and vids, etc). This shows up when anyone clicks on your profile or when you view your own profile as others see it.